This invention relates to a double-cylinder selector valve for detecting and locating fuel assemblies including failed nuclear fuel pins within a nuclear reactor.
When the cladding of nuclear fuel fails, particles of nuclear fuel and fission products are emitted into reactor coolant, contaminate the nuclear power plant, and will eventually cause radioactive pollution and problems for the operator of the reactor and the people in the surrounding environment. It is therefore necessary to rapidly detect the occurrence of any failure in the cladding of fuel and locate which fuel assembly has the failed cladding.
A system using a selector valve shown schematically in FIGS. 1 and 2 is known as a typical conventional apparatus for detecting the location of a failed fuel assembly in a fast breeder reactor. In the selector valve device 1, a thin sampling pipe 2 is disposed at a coolant outlet of each fuel assembly (not shown) so as to separately sample part of the liquid metal coolant flowing out from each fuel assembly. The ends of these sampling pipes 2 are gathered at one position inside the reactor, and are arranged to open circumferentially around the periphery of a sliding plate 3. A rotary pad 6 is slidably provided on the sliding plate 3 and is rotated by a motor 5 via a rotary shaft 4. The rotary pad is connected to a delayed neutron detector 9 by a tube 7 via an electromagnetic pump 8. As the rotary pad 6 rotates slidably around the periphery of the sliding plate 3, a particular sampling pipe 2 opened at the periphery of the sliding plate 3 is selected and part of the liquid metal coolant flowing out from the corresponding coolant outlet of the fuel assembly is sampled. When a fuel failure occurs, fission products are emitted into the coolant. The fission products contain delayed neutron precursors that emit delayed neutrons and disintegrate. These delayed neutrons produced as a result of the decay of the precursors are detected by the delayed neutron detector 9, thereby detecting and determining the location of the failed fuel assembly.
However, in the conventional system described above, the valve portion (sampling switching portion) is arranged in a plane so that as the number of fuel assemblies increases, the installation space needed within the reactor increases, causing problems concerning the load balance of the rotary pad and a complicated structure in the space above the reactor. Moreover, since the system has a construction in which a sliding surface is pushed from above, the whole pad portion must be replaced every few years; and since the construction is complicated, disassembly and repair is difficult when a fault occurs.